This invention relates to a free radical polymerization in the presence of an oxygen source and a select class of organoboron catalysts.
The initiation of polymerization of vinyl monomers using trialkylboranes in the presence of oxygen, air, peroxide, or other oxygen source is known. One of the advantages of organoboron initiators is ability to initiate polymerization at low temperatures. Peroxides and azo initiators when used alone usually require considerable heat input to decompose and thereby to generate free radicals. Elevation of the temperature, however, often causes significant reduction in molecular weight of the polymer accompanied by loss of important properties of the polymer. Representative of the prior art which discloses the use of organoboron catalysts for vinyl polymerization are U.S. Pat. Nos. 3,476,727, 3,633,490, 2,985,633 and British Pat. No. 1,113,722.
Despite advances in the art as represented by the foregoing prior art, organoboron catalyzed polymerizations tend to be unduly sensitive to the concentration of oxygen in the polymerization system. Too little oxygen results in little or no polymerization because it is believed that at the low concentrations of trialkylboranes present in such systems, the bimolecular reaction between trialkylborane and dialkylalkylperoxyborane (the latter being produced by reaction of trialkyl borane with oxygen), required for production of free radicals and initiation of polymerization, is unduly slow. If too much oxygen is present, little or no polymerization will occur because all of the trialkyl borane is rapidly transformed to dialkylalkylperoxyboranes which will initiate polymerization only at elevated temperatures. Moreover, polymerization is often inhibited by oxygen.
The foregoing reactions are illustrated by the following equations where R is an organic radical such as n-butyl: EQU (R).sub.3 B+O.sub.2 .fwdarw.(R).sub.2 BO.sub.2 R (1) EQU (r).sub.2 bo.sub.2 r+(r).sub.3 b.fwdarw.[(r).sub.2 b--o.]+(r).sub.2 bor+r. (2)
because the organoboron catalysts of the invention contain at least two boron atoms, the bimolecular reaction between trialkylborane and dialkylalkylperoxyborane and resultant production of free radicals is greatly facilitated by neighboring group participation and is largely insensitive to oxygen concentration and reaction temperature.
The following equations illustrate the formation of free radicals from the organoboron catalysts of the invention, where the diborane catalyst typically is the adduct of borane (BH.sub.3) and 1,3-butadiene (prepared in tetrahydrofuran, THF): ##STR1##